This invention relates to a pavement marking sheet material and, more particularly, to a high-brightness pavement marking sheet material having a high initial brightness in reflection and enjoying timewise continuity in the brightness in reflection.
There are various types of prior art sheet materials used for pavement marking purposes in a state in which they are bonded to the pavement surface.
Typical ones of these prior art pavement marking sheet materials are of a generally flat sheet surface and can be classified into four types: namely (1) those having a monolayer of glass microspheres partially embedded in the base sheet and partially exposed from the surface of the base sheet to the air (e.g. the specification of U.S. Pat. No. 3,915,771), (2) those having a monolayer of glass microspheres partially embedded in the base sheet and partially exposed from the surface of the base sheet to the air and further having glass microspheres dispersed and completely embedded in the base sheet (e.g., the specification of U.S. Pat. No. 3,030,870), (3) those having two layers of a support sheet and a base sheet and having a monolayer of glass microspheres partially embedded in the support sheet and partially exposed from the surface of the support sheet to the air (e.g., the specification of Japanese preliminary patent publication No. 3707/1981 and the specification of U.S. Pat. No. 4,248,932) and (4) those having two layers of a support sheet and a base sheet and having a monolayer of glass microspheres partially embedded in the support sheet and partially exposed from the surface of the support sheet to the air and further having glass microspheres dispersed and completely embedded in the base sheet (e.g., the specification of U.S. Pat. No. 4,117,192).
The above-described prior art pavement marking sheet materials have the common drawbacks that an amount of glass microspheres which can be dispersed on the surface of the sheet is restricted because the employment of an excessive amount of glass microspheres causes slippage of vehicles running over the sheet and that, when light is incident at a large angle to a normal of the surface of the sheet i.e., obliquely with respect to the surface of the sheet, an amount of reflection back towards the light source is greatly reduced with a resulting sharp decrease in the brightness in reflection. Further, these prior art sheet materials have the drawback that they lack in timewise continuity of reflection of light because, after the layer of glass microspheres on the surface of the sheet has been lost due to collision with wheels of the passing vehicle, the sheet materials of the above-described types (1) and (3) entirely ceases reflection of light and those of the above-described types (2) and (4) take a considerable length of time before glass microspheres of a next layer become exposed from the surface of the sheet so that the sheet scarcely reflects light or reduces its brightness in reflection during this period of time.